The present invention relates to a method for detection of a pilot signal provided from a base station in a mobile communication system employing CDMA (Code Division Multiple Access) for example and required for demodulating a signal from the base station, and also relates to a radio communication terminal apparatus adopting such a detection method.
In communications where a multiplicity of mobile stations share a single base station in common, it is generally customary to employ a technique of frequency division multiple access, time division multiple access or code division multiple access in order to avoid interference between the communication channels of the mobile stations. Each of such techniques has individual characteristics respectively, and an appropriate one is selectively employed in compliance with the purpose of the communication system.
For example, in code division multiple access (hereinafter referred to as CDMA), a specific code assigned to each channel, e.g., PN (pseudo-random noise sequence) code, is used for spreading modulated waves of one carrier frequency to another band wider than the original frequency band (hereinafter referred to as spread spectrum), and the modulated waves processed by such spread spectrum are multiplexed and transmitted. And the received spread spectrum signal is synchronized with the PN code supplied via a subject demodulation channel, whereby only the desired channel alone is identified.
More specifically, first on the transmission side, different PN codes are assigned to the channels individually. In this case, the PN codes are composed of pseudo-random noise sequences. On the transmission side, modulated waves to be transmitted via the relevant channels are multiplied by mutually different PN codes to be thereby processed through spread spectrum. In this stage, the modulated waves to be transmitted have already been processed through predetermined modulation prior to such spread spectrum. The modulated waves thus processed by spread spectrum are then multiplexed and transmitted.
Subsequently on the reception side, the received signal sent from the transmission side is processed through inverse spread spectrum by synchronous multiplication of the same PN code as that assigned to the subject demodulation channel, so that only the modulated waves transmitted via the desired channel alone can be demodulated.
According to such CDMA, communication can be performed directly per call only if the mutually identical code is preset on both the transmission and reception sides. In the CDMA where a process of spread spectrum is executed for modulated waves by the use of different PN codes with regard to the respective channels, the reception side is capable of demodulating only the spread spectrum signal transmitted thereto via the subject demodulation channel, and superior maintenance of privacy can be achieved since the PN codes are composed of pseudo-random noise sequences.
Also in the mobile communication system employing such CDMA, each base station on the transmission side sends PN codes repeatedly as pilot signals for attaining synchronism in the mobile stations, keeping the synchronism and further for reproducing clock pulses. And in each mobile station on the reception side, the pilot signals sent from a plurality of base stations are detected, and the timings of detection are allocated to the individual demodulators respectively. Each mobile station generates a PN code in its demodulator. The demodulator then demodulates the spread spectrum signal, which is transmitted from the desired base station, through multiplication by the generated PN code at the allocated timing.
Thus, in the CDMA mobile communication system, the base stations send PN codes of mutually different timings as pilot signals. Each mobile station detects the timing of the pilot signal supplied from the desired base station, then synchronizes the PN code, which is generated in its demodulator, with the detected timing, and executes inverse spread spectrum by the use of the PN code as mentioned, thereby properly demodulating only the spread spectrum signal transmitted from the desired base station.
Although the base stations transmit PN codes of mutually different timings as described, the PN codes themselves are in the same code pattern. That is, the timing difference between the different pilot signals of the individual base stations corresponds directly to the difference between the PN codes of the base stations.
In the mobile communication system, any mobile station such as a radio communication terminal is normally placed in a service waiting state after being switched on and synchronized with the system. In this case, a mobile communication service denotes a call or the like rendered by telephone service. The mobile station settles a call to receive a mobile communication service and, after completion of the service, disconnects the call to be thereby placed in a service waiting state again.
After such disconnection of the call, it is desired that the mobile station be reset immediately to a state ready for receiving a mobile communication service again. For example, according to the CDMA standardized as IS-95 in U.S.A., it is prescribed to receive the same CDMA system after the mobile station disconnects a call. More concretely, it is prescribed to receive a pilot signal of the same frequency provided by the same CDMA system managerial proprietor.
As for a method of detecting a pilot signal after disconnection of a call, it may be considered to employ the same pilot signal detection method as that used when switching on the mobile station. In this case, for detection of the pilot signal, an integration of 128 chips is executed, and one period (32,768 chips) of a PN code is detected. Supposing now that one chip corresponds approximately to 0.8 xcexcsecond as prescribed in the IS-95, then the time required for detecting the pilot signal is
128 chipsxc3x9732,768 chips=4,194,304 chips=Approx. 3.4 secondsxe2x80x83xe2x80x83(1)
Thus, due to such detection of the pilot signal from the base station after disconnection of each call in the same manner as in switching on, the mobile station fails to be placed immediately in a waiting state and needs a lapse of approximately 3.4 seconds after disconnection of each call, so that no service is receivable during that time. For this reason, there is demanded an improved pilot signal detection method which enables any mobile station to receive a pilot signal fast and efficiently from the base station in the same CDMA system and further enables the mobile station to be reset quickly to a state ready for receiving a mobile communication service.
It is therefore an object of the present invention to provide a pilot signal detection method which enables a mobile station to receive a signal fast with high efficiency in the same CDMA system after disconnection of each call and is capable of resetting the mobile station to a waiting state ready for accepting the provision of a mobile communication service.
And another object of the present invention resides in providing a radio communication terminal apparatus adopting such a pilot signal detection method.
According to one aspect of the present invention, there is provided a pilot signal detection method carried out in a CDMA radio communication terminal apparatus. The detection method comprises the steps of detecting, during connection of a channel, the received energies of a plurality of pilot signals transmitted from a plurality of base stations, then monitoring and storing the detected energies of the plural pilot signals and, when connecting the channel again after disconnection thereof, detecting the timings of the pilot signals preferentially in order from the pilot signal of the greatest received energy on the basis of the result of monitoring the stored energies of the plural pilot signals.
According to another aspect of the present invention, there is provided a CDMA radio communication terminal apparatus which comprises a detection means for detecting, during connection of a channel, the received energies of a plurality of pilot signals transmitted from a plurality of base stations; a monitor means for monitoring and storing the detected energies of the plural pilot signals; and a control means for controlling the detection means in such a manner that, when connecting the channel again after disconnection thereof, the timings of the pilot signals are detected preferentially in order from the pilot signal of the greatest received energy on the basis of the result of monitoring the stored energies of the plural pilot signals.
Thus, in detecting a new pilot signal after disconnection of each call, the base-station pilot signal of the greatest received energy can be preferentially searched on the basis of the result of monitoring the received energies of the pilot signals from the base stations prior to disconnection of the call, hence realizing fast and efficient re-acquisition of the pilot signal after disconnection of the call.
The above and other features and advantages of the present invention will become apparent from the following description which will be given with reference to the illustrative accompanying drawings.